1. Field of the Invention
The present invention relates to a method and an apparatus for rolling strip in a strip rolling train having at least two roll stands, wherein each roll stand has horizontally adjustable upper and lower work rolls which either act alone (two-high stand) or which are each supported directly or through an intermediate roll by a back-up roll, or with a reversing stand in which at least two passes are rolled, wherein the strip is subjected in the roll stands to a structural or constitutional change, and wherein adjusting elements act on the strip for imparting a profile and surface evenness to the strip.
2. Description of the Related Art
In practice, continuously increasing demands are made of the surface evenness of the hot strip as well as of the surface evenness of the cold strip. Simultaneously, the boundary conditions for hot rolling become more and more difficult because increasingly thinner and wider products are demanded, which leads to greater reductions and increased rolling forces even in the rearward stands, i.e., the stands on the exit side of the train. With increasing reduction, the wear increases (CSP-plants) and the thermal crown increases with increasing train production, i.e., in the case of endless rolling or in a hot aluminum strip rolling train.
This means that there is an elementary technical and economical demand to maintain through an optimum presetting an improved evenness of the strip even under extreme boundary conditions and there is a corresponding demand for an improvement of the surface evenness of a rolled strip even within the train; this demand is dependent on the purpose of use and dependent on the processes to be taken into consideration when the hot strip is cooling on the run-out roller table and in the coil.
When hot rolling strip material, the thermal crown or camber and the wear of the work rolls and the elastic deformations thereof are subject to relatively great changes within a rolling program. Without carrying out corrections by means of adjusting elements, the roll contour changes within increasingly continuing rolling stock throughput. The effect is different from stand to stand and from pass to pass. Consequently, in addition to the strip contour, the predetermined surface evenness of the hot strip and, thus, the surface evenness of the cold strip also change.
When rolling in one width, a number of strips with equal widths or approximately equal widths are rolled successively within a rolling program. Simultaneously with the value of the strip profile predetermined for a certain point (for example, C.sub.40 or C.sub.25), the total shape of the strip profile changes and, thus, the predetermined surface evenness of the strip in the middle areas of the strip as well as especially also in the areas of the strip near the edges also changes. For example, the increasing drop of the thermal crown of the rolls or the wear of the work rolls in the areas near the edges result in undesired profile anomalies. These anomalies are increased thickness areas at the edges (beads), or, in contrast, decreased thickness areas at the edges. These types of profile anomalies substantially limit the rollable length within one width. In this regard, the rolling length within one width is defined as the sum of all strip lengths which are rolled in one width or with approximately equal widths.
DE 30 38 865 C1 discloses that changes of the thermal crown and of the wear of the work rolls can be compensated by means of suitable adjusting elements, such as displacement elements and/or bending elements, for example, continuously variable crown displacement, or by means of a suitable cooling.
For controlling the convexity and/or the edge drop of the strip, EP 0 276 743 B1 discloses adjusting the horizontal displacement of the work rolls and the bending forces acting on the work rolls of a group of roll stands located on the upstream side of a tandem rolling mill in accordance with the rolling conditions including the widths of the strips.
For controlling the wear and the thermal crown of the work rolls with the object of avoiding undesired profile shapes and surface unevenness, the work rolls of a group of roll stands on the downstream side are pushed back and forth within predetermined intervals. In this case, the rearward stands are displaced after each strip in opposite directions by a certain distance; once the displacement amount has reached a maximum value, the displacement direction is reversed. This cyclical displacement causes the wear of the work rolls to be uniform over a larger area.
EP 0 618 020 A1 discloses a method of rolling strip in a roll stand of the above-mentioned type with horizontally adjustable upper and lower work rolls, wherein adjusting elements act on the strip for influencing the profile and surface evenness of the strip. The known rolling method makes it possible in spite of flexible rolling programs to approximately meet the requirements with respect to profile accuracy and surface evenness of the strip when an intended or target contour of the profile of the strip is predetermined, wherein for achieving the intended contour successively two groups of adjusting elements act on the strip, and wherein a first group of adjusting elements are used and primarily the contour of the strip in the middle area thereof is influenced when the strip thickness is above the critical thickness, while a second group of adjusting elements are used when the strip thicknesses in the edge areas thereof are below the critical thickness.
However, the measures known in the prior art are not sufficient for meeting the increased demands particularly with respect to the surface evenness even under extreme boundary conditions. When producing hot strip, these boundary conditions concern the flexible setting up of rolling programs, wherein, in addition to increased thicknesses and material changes, especially with changes in the direction of narrow and wide are desired (mixed rolling). In addition, for economical reasons, the number of strips of equal width are to be increased within one rolling program without impairing the profile accuracy and surface evenness.